Elimination of interference in carrier systems



Patented July 7, 19.25.

n Y, l 1,544,137y PATENT g o1-"FICE,

Es'rILL 1. GREEN, or EAST ORANGE, NEW JERSEY, AssIGNoE lrro.AMERICANI.'rELE- -rHoNE AND TELEGEAPH- COMPANY, A CORPORATION vor NEW YoEK.

ELIMINATION OF INTERFERENE IN CARRIER` SYSTEMS.

Application filed Mayl 1o,

To ZZ/whom z'may concern.'

Be it known that I7 EsTrLL I. GREEN, residing at East Orange, in the county of Essex and State of New Jersey, have in- 'vented certainA Improvements in Elimination of Interference in Carrier Systems, ofl which the following is a specification.

This invention relates to carrier systems and more particularly tothe elimination of external interference in such system. In systems employing carrier frequencies for the transmission of signals, one of the principal sources of external interference is due to radio transmitters employing-fre quencies lying Within the rangetransmitted bya carrier channel. vSuch 4interference is produced by the line wiresl acting as an antenna to pick up vthe radiated wave and transmit it to the lreceiving carrier apparatus. i

It has heretofore been proposedto overcome interference of this sort by picking upl the vinterfering wave and impressing it upon the circuit yat some point. common to all j ofthe carrier channels, with such phase and amplitudeas to neutralize the interfering component transmitted over the line wires". While this method' is quite. satisfactory where only .one interfering wave is involved, difficulty has been experienced where, as is usually the case, there are other interfering sources, and particularly where the circuit is 'subject to noise disturbances. In the case of a-noise interference, such as that due to a power line, for example, the frequency and amplitude are quite'variable and while the fundamental noise currents themselves are usually within theordinary telephoniev range, there are harmonics which `fallv within the carrier range.` U y j c These harmonics, when they lappear. inthe circuits as interference, areusuallylof Siich'- small amplitudev that they' may loev neglected; But where a selective circuit is provided for selecting and then changing thekphase and amplitude relation of the wave yfrom a delle nite interfering source, such as a radio sta-` tion, the noise or other interfering kcurrents will also be selected to some extent and will be transformed in amplitude and phase re lation. If, then, ythese currents are' 'iin'- lpressed upon vthe carrier circuit at a point common to the/channels, while the component corresponding tothe radio wave will be .l such vamplitude and phase as tobaflancev 1924. vsmal No. 712,407.

transmitted over the line; and thusrincreaseI theinterference from'. theseV sources. y In accordance with the presentv invention it 1s proposed to overcome thisdifliculty by im' ,applying the component'for neutralizing` a particular interfering wave to the desired channel at a point'beyond the band filterl which is individual'to thatl channel. As a. consequence, anyvnoi'secomponents intro- 70 duced into the, circuit will onlya'flect that particular channel yand cannot add to the noise components transmitted over the lineand passing i'nto other channels.l

The Vinvention may now be more fully 75 understood from thefollwing description when 'read in connection with the attached drawing, Figs. 1 and 2 of which illustrate two embodiments of the invention;

Referring to Fig. 1, ML designates the main ltransmission line of `a carrier system, said line being connected through a vtransf former l0 to the common receiving circuit RL ofcarrier terminal apparatus. The re-y ceiving channels, such as RL1 and RLZ, are 85 connected to thefcoinlnon receiving channel RL and include band filters, such as RF1 and BF2 for selecting intojeach channel the proper band of frequencies. Demodulators such as D1 and D2, of any well-known type, im' `are also included in the channels for trans' lat-ing the carrierfrequencies into signal-'l ing currents.` s' 1 IJet us assumethat two 'radio 'stations are .transmitting,*one` within' the frange 'of the Q5 filter4 RF1` and' the other `Within therangevof' i' ,the 'fltorRF, 'and' that these radio stations vare l'so' located' that "undesirable inter'fe'rencefy results. I vThe interfering s waves from"y thelf'z radio,stationwillllow overv the two sides of' 100,

thema-in' liney ML,*inparallela-but" due to f thedissilnilarity of thetwdy sides of the line an unbalanced component will result, which"l tends' to ,flow 'serially'ov'e'r the twosides of i "the" line." The unbalanced component of fthe @105, i

"two *radio waves--' will* produce vinterference 'i in the receiving channels RL1 and RL2 unless suitable measures are taken. By bridging an inductance element 11 across the conductors of the main line ML and taking a 1w tap, such as 12, 4from the midpoint of the bridge 11, the wave component flowing over the two sides of the line in parallel (which does not cause an interference) may be rcceived in a sutab e circuit and so translated as to neutralize the interferingcomponent, which tends to iiow serially over the line;

One of the interferin components will, of course, flow into the c annel RL1 and in order to neutralize this component a branch ceiving channel RL1 in opposite phase rela-v tionv to the 1nterfering component transmitted through the filter RF1. A change in phase may be made without materially changing the frequency at which the circuit is resonant, for the phase change is very large in response to a slight adjustment of the condenser at the resonance point.'

Similarly, for the other interfering component entering the channel. RL2 a branch 22 may betaken from the conductor 12 and the inductance L2, "capacity C2 and resistance may be adjusted tovproduce anelec* troinotive force through the transformer 32, Which opppses and neutralizes the interfering electromotive force passing through the filter BF2.

It will be noted that the neutralizing lelectromotive forces are applied to the :individual receiving channels at points beyond the band filters, such as R1 and R1, which select the desired bands into the channels. This is an important feature of the present invention for it overcomes, to a large ,extent7 one of the difficulties encountered in inter' ference arrangement-s heretofore employed. For example, let us consider the tuned4 circuit 21. Although this circuit is tuned to the frequency of the particular interfering Wave with which we are concerned in the case of the channel RL1', it cannot be made to absolutely discriminate against the frequency to which the circuit 22 is tuned. Consequently, a component of the other interfering frequency will also appear in the circuit 21.' The adjustment of the capacity C1 to produce the desired phase shift 0f the particular interfering component entering the channel RL1 will also produce a phase shift in the component of the other interfering 'frequency which enters the branch 21.- If,'now, the branch 21 is con-` the vneutralizing component for the interfering frequency which enters the channel RL1, Will not produce the right phase rela* tion for the other interfering frequencies,

Sil

and it may very Well happen that the energy',

of the other interfering frequencies inn pressed upon the common point of the circuit vvill be insuch phase relation as to actually add to the other interfering frequency.

The same holds true for harmonics of noise Waves upon the circuit. Owing to the irregular nature of the noise Wave it is not readily possible to balance an interfering noise by producing another Wave of Oppo site phase. Now, obviously, some of the harn monies due to noiseWaves will lie Within the regions of both carrier channels and some of these noise components will enter both the circuits 2l and 22, notwithstanding the tuning of these circuits. These noise components, if impressed by circuits 21 and 22 upon the common part of the circuit such, forexample, as the transformer 10, will not be in such phase relation. as to neutralize the original noise components but will add to and increase the effect of the noise Waves in the several channels.

On the other hand7 by impressing the neu tralizing electromotive force upon the ndividual channels beyond the band filters in accordance With the present invention it Will be clear that any interfering components falling Within the range of the band filter RF2 and which are able to enter the branch tuned circuit 2i cannot be cumulativelyfV effective with rthe original noise components in the channel RLZ, for the balancing components will be applied to the channel RLl beyond the filter RF1 and can other wire of cach channel being connected y lll each channel, an arrangement for neutraliz- ,components comprising to ground through the inductances, capacities and resistances, such as L1, C, and R1. The principle of the arrangement shown in Fig. 2, however, is the same as that of Fig. l, yand no yfurther description is deemed necessary. l

It Will be obvious that the general principles herein disclosed may he embodied in many other.organizations widely different from those 'illustrated without departing from the spirit of the invention as defined in th following claims.

What is claimed is:

l. In a carrier system comprising'a main line and a plurality of branch 'receiving channels, each havingA a filter therein forv selecting a desired range of .frequencies for each channel, an arrangementfor neutralizing an interfering component comprising means for receiving energy corresponding to the interfering component, means for producing from said energy a wave 'equal in' amplitude but opposite in phase to the inf terfering component, and means for impressing tlie wave thus translated upon the channel-interfered with at a point beyond the vfilter in that channel whereby-the balancing wave will be prevented from react ing in other channels.

2. In a carrier system comprising a main line and a plurality of lloranch receiving channels, eachhaving a selecting a desired range of frequencies for each channel., an arrangement for neutral'- izing vinterfering components comprising fneans for receiving energy corresponding to the interfering components, means for separating the components falling into the -range of each receiving channel, and means for impressing upon each channel the separated components corresponding to the beyond. the filter in such phase relation a tov neutralize the interfering wave transmitted by that channel through the filter.

3. ln a carrier system comprising a main line and 'a 'plurality channels, each having ajiilter therein for selecting a desired range of frequencies for ing 1 interfering means for receiving energy corresponding to the 'interfering components, means for filter therein for range of that particular channel at a poing'.

'of branch receiving separating the components falling into the range of each receiving' channel, means for impressing upon a given channel the :sep-l arated components corresponding fo that channel, and means for adiusting the phase and amplitude of the energy thus impressed so that it willbe equal in amplitude and op. posite in phase to the interfering componente entering the channel.

4. In a carrier system comprising a main line and a plurality vof branch Areceiving channels, each having a filter therein for selecting a desired range of frequencies for each -channel, arrangements for neutralr izing the effect of interfering Wave com ponents flowing serially with respect to the line wires of thesystem, said arrangements comprising a bridge connection for the line, a conductor connected to a midpoint of the connections being made at the points beyond the filter in the channel.

5. In a carrier system comprising a. main bridge to take off components of the inter- -line and a plurality of branch receiving channels, each having a filter therein for selecting adesired range of frequencies for each channel, arrangements for, neutralizing the effect of interfering wave components flowing serially with respect to the line wires of the system, said arrangements comprising a bridge .connection for the line, a'. conductor connected to a midpoint of the bridge to take off' components of the interfering wave flowing over the two sides of the line in parallel, branches for said conductor, each tuned to edifici-ent interfering frequency, `connec from each vof sai branches tothe channel within whose range 2th Ifrequency of the branch lies,

ions ioeing made 'atl points beyond the Irs inthe channels, land means for ad- :justing the phase and amplitude of the currents will be impressed upon corresponding channels with equal amplitude and' in opposite phase relation to the interfering comp nents passing into the channels through the filters. In testimony whereof, I have signed my name to this specification this 17thday of April 1924. f-

flowing in the branches sp that they said .con- 

